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Full text of "Text Book Of Mechanical Engineering"

Appendix I,

volume at 70 Ibs. pressure, and at JD F that due to 30 Ibs. pressure,
Through E and F respectively dra\v the saturation curves E G and
F H, which for rough purposes may be hyperbolas with origin o;
but, for greater accuracy, may be drawn with a new origin for
each curve, according to the inarmer described on p. 624, and
shewn in Fig. 624. Thus JE being divided into 6'i parts, and
K F into 13*5 parts to find the scale of specific volume, the origin
for each curve must be moved "41 ft. to the right and '35 lb.
below the old origin, and the curves are then treated as
hyperbolas from the new origins. The method of constructing an
hyperbola is shewn at p. 615.

It will be seen that there is a separate saturation curve for
each cylinder of the compound engine, but this is only due to the
fact that the clearance volume in the L.P. cylinder does not bear
the same relation to the steam volume as did that in the H.P.
cylinder. If it be desired to shew both cards under one
saturation curve, it is only necessary to transfer the lower card
to tracing paper and move it to the right until the curves E G and
F H coincide. Or another method is to re-set out the cards by
placing the curved lines c N D and A M B upon the vertical line
o K j, and so move the cards to the left, distorting them some-
what. One saturation curve may then-be drawn through E, B E
being the same as before, for the clearance steam is now
eliminated. Of course, instead of completing the compression
curves by the dotted lines, the curves could have been drawn for
any pressures between A M in the H.P, cylinder, and between c N
in the L.P. cylinder.

Assuming now that the cushion steam is dry—^n,, assumption
involving but very slight error—the dry ness of the steam may be
found for any point in the stroke of either piston, fcy comparing
the total volume of steam in the^ cylinder with that shewn by the
saturation curve at that point, the difference of these volumes
shewing the amount of steam condensed by the cold cylinder
walls. Thus, at commencement of the H.P. stroke:—

JQ
Dryness fraction =» —

Q E

Wetness fraction = —
j E